Method and apparatus for treating fluent materials

ABSTRACT

A process for heating fluids to a relatively high temperature, such as sterilization temperature, in which the fluid, such as a liquid, is heated by direct contact with steam while it is in the form of a very thin, free-falling film or a continuous falling stream so that heating of the fluid is accomplished without the fluid coming into contact with any surface and particularly metal surfaces which are hotter than the fluid product being heated and with minimum agitation and turbulence of the fluid product. This procedure enables maximum and uniform heat penetration in a minimum time interval with the film or stream being maintained as thin as possible and unbroken by introducing steam at a relatively low velocity in a large volume vessel. A flow control is incorporated into the apparatus to maintain a constant flow rate of fluid and to maintain a constant and critical fluid level in the bottom portion of the large volume vessel with the flow characteristics of the apparatus and the internal forces produced by the apparatus serving to counterbalance each other to provide a relatively simple but yet accurate flow rate and liquid level controls for the fluid being heated.

RELATED APPLICATIONS

This is a continuation of application Ser. No. 083,362 filed Oct. 10,1979 now U.S. Pat. No. 4,310,476. Ser. No. 083,362 is acontinuation-in-part of application Ser. No. 29,391 filed Apr. 12, 1979which is a continuation of application Ser. No. 806,849, filed June 15,1977 both now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process and apparatus for sterilizingfluent materials without disturbing the natural flavor and stability ofthese materials. The invention has particular application to fluent foodproducts.

2. Description of the Prior Art

The science of preserving food products has been studied continuouslyfrom the stone age to the present day. The application of heat to foodproducts as a preservation technique was probably first used to cook anddry meat in stone age human societies. A major step forward in heattreatment of fluid food products was made in the 19th century with thedevelopment of pasteurization, a process of partial sterilizationinvolving subjectng a substance, particularly a liquid, to a temperaturefor a period of time that destroys disease-causing organisms withoutmajor chemical alteration of the substance. Numerous other techniqueshave been developed more recently wherein fluent food products arecompletely sterilized to eliminate bacterial spoilage and permit storagewithout refrigeration. However, the affluent consumers of modern foodproducts do not view the use of preserved foods simply as a technique ofstaving off starvation, but rather have the option to choose the mostappealing food products at will. Thus the factors that make foodproducts appealing to modern consumers have become the most criticalfactors to be observed in food processing and preservation. The mostcrucial of these factors without doubt are taste and convenience. Ofthese two factors taste is perhaps paramount although convenience isbecoming more and more critical, especially as it relates to energyconservation.

Referring first to the concept of taste, it is believed clear that ifmodern consumers are presented with two preserved food products, one ofwhich tastes the way they expect it to taste, and the other of whichtastes even slightly different from the way they expect it to taste, thefirst product will attain wide commercial success, while the secondproduct will be saleable, if at all, only if its cost is significantlyless than that of the first product. Proper taste is an especiallycritical factor in products such as milk to which virtually everyone isexposed during his lifetime. Nearly every consumer has tasted milk andknows exactly how it should taste. In many cases consumers have alsotasted sour or slightly sour milk and various forms of fully-sterilizedor processed milk. Due to such wide-spread and often life-longexperience, consumers develop an acute sensitivity to taste variation inmilk products. Similar circumstances apply, although to a lesser degree,to other common products such as orange juice, beer, selected types ofsoup and the like, although milk as a product that one experiencesvirtually from birth, is a matter of particular sensitivity toconsumers. Thus a major technical problem that has nagged the dairyindustry from its inception is the development of a technique for fullysterilizing milk without perceptibly changing its taste. Although theindustry has actively researched this problem since before the beginningof the twentieth century, every solution which has been proposed hasfailed due to the complex nature of milk itself and due to the highsensitivity of the consuming public to slight variations in the taste ofsterilized or processed milk.

The second previously mentioned factor of major concern to moderconsumers is that of convenience. Consumers are willing to pay asubstantial premium for food products which are particularly convenientto use or store. Consumers are particularly well aware of the shortshelf life of milk and the need to keep it refrigerated. Accordinglythey customarily make far more trips to market for the purpose ofpurchasing milk than for any other reason, according to studiesconducted by the U.S. Department of Energy. Such additional trips tomarket for the purpose of purchasing only one product are becoming moreand more burdensome and inconvenient with the increasing cost anddecreasing availability of automotive fuels. Furthermore, the need tocontinuously refrigerate fresh milk creates additional inconvenience andloss of economy. Refrigeration requires substantial energy and milk,normally being a bulky product, consumes a large portion of personal andcommercial refrigerator space. The cost of fresh milk is raised by theextensive refrigeration energy expended by dairy producers, wholesalersand retailers of the product. Accordingly fresh milk as it is presentlyknown and utilized is a product that creates considerable inconveniencein requiring numerous otherwise unnecessary trips by consumers to retailestablishments and by the fact that continuous refrigeration isrequired. Both of these undesirable factors could be eliminated if fullysterilized milk were available. Such a product would have an extensiveshelf life and would not require refrigeration so that consumers couldpurchase large quantities of sterilized milk at regular intervals forstorage without refrigeration. Similarly, wholesalers and retailerscould also store large quantities of the product without refrigeration,thereby reducing the overall cost of the material to the consumer.Consumers would experience multiple savings in utilizing a sterilizedmilk product in the form of fewer trips to the store resulting in lessfuel consumption and, in some cases, the need for fewer automobiles perhousehold as well as in possibly reduced energy costs due to thepossibility of utilizing smaller refrigerators.

While sterilized milk clearly possesses a number of advantages from thepoint of view of convenience and energy saving, the problem of itsproduction without substantial taste distortion relative to fresh milkhas prevented sterilized milk from gaining a substantial foothole in theconsumer market. It is the complex chemistry of milk which makes itparticularly subject to changes in taste upon heat treatment. To fullyunderstand this taste sensitivity of milk to heat treatment, it isbelieved that a brief summary of milk chemistry is in order.

It is well known to those skilled in the art that milk contains amongits various constituents the following nutrient items:

Water

Proteins, such as casein, lactalbumin, lactoglobulin

Vitamins

Gases

Milk fat

Lactose (sugar of milk)

Milk ash

Pigments

Enzymes

Cellular material

Each of these nutrients reacts differently upon exposure to varioustemperature ranges for selected time intervals. Thus any heat treatmentof milk must take into effect the characteristics of these nutrients aswell as other organisms such as bacteria, spores, yeast and mold presentin non-sterile milk. Unfortunately, all of the relationships between thevarious elements constituting milk are not fully understood, even bythose highly skilled in the art of milk chemistry. Thus it is only byexperimentation with new techniques for sterilizing milk that a processand apparatus can be developed wherein sterilized milk is produced butstill retains all of the desirable qualities and characteristics offresh milk such as flavor, stability, body and color.

As a result of extensive experimentation, Elmer S. Davies and Frank D.Petersen developed a series of time-temperature relationships and ageneral technique which appeared promising in the development of sterilemilk which maintains all of the desirable qualities of fresh milk. Thisdevelopment is disclosed in U.S. Pat. No. 2,899,320 (Davies et al),issued Aug. 11, 1959. As is pointed out in this basic patent, to betruly effective in producing a sterilized milk that retains all of thedesirable characteristics of fresh milk, a considerable number ofindependent reactions must be either accomplished or avoidedsimultaneously. Specifically, living organisms must be completelysterilized and enzymes inactivated. However "browning" and coagulationmust be avoided. "Browning" is due to the heat sensitivity of lactoseand casein as present together in milk. Similarly, coagulation is afunction of temperature resulting from the combination of casein, milksugar and whey in the protein content of the milk. Coagulation resultsin an undesirable increase in the viscosity of the milk and produces an"off" flavor which is highly objectionable and easily detectable byconsumers. Furthermore, the release of sulfhydrils in the course of theheating process produces a "cooked" flavor in heated milk. Sulfhydrilsare formed by the breakdown of the whey portion of milk proteins,particularly the beta lactoglobulin upon heat treatment of milk.

In the above-referenced Davies et al patent, the followingtime-temperature relationship was established as the most effective inattaining full sterilization of milk with minimum effect upon itsdesirable characteristics: heating to approximately 300° F. for 1.5 to3.0 seconds. Further experimentation has shown this relationship to havea temperature range of approximately 280°-310° F. and a time range ofapproximately 1.5 to 9.0 seconds. While this time-temperaturerelationship still remains optimum, it has since been discovered thatmore subtle factors are involved in maintaining the flavor of sterilizedmilk sufficiently close to that of fresh milk that consumers cannotdetect the difference. These factors involve the extent of physicalagitaton or perturbation experienced by the milk during heating, theuniformity of heating and the extent to which the heated milk contactssurfaces hotter than itself during or subsequent to the heatinginterval. Furthermore, proper cooling and handling of the milk prior toand subsequent to heating have also been found to be a factor inmaintaining taste perfection in sterilized milk. Because of theinteraction of these many factors during the sterilizing process,extraordinarily sophisticated constraints have been imposed upon anysterilization system which is to be successful in maintaining perfectflavor quality during sterilization. The lack of detailed knowledge asto the effects of these very subtle factors on milk flavor has severelyimpeded the development in the prior art of any successful device ormethod which could successfully produce sterilized milk having a tasteacceptable to the highly refined standards of modern consumers.Furthermore, the unique combination of constraints, once known, hassubstantially defeated prior engineers and researchers in their effortsto construct a truly satisfactory milk sterilizing apparatus.

Experimental studies conducted by Elmer S. Davies and Frank D. Petersen(see Davies et al) led to the conclusion that the risk of denaturationof milk proteins was reduced if sterilization was conducted at highertemperatures than previously used, but for shorter time intervals. Theconcept of heating milk to a high temperature for a short time led tofurther studies to determine how such heating could be mostadvantageously accomplished. It was eventually determined that a fallingfilm of product provided the optimum configuration for attaining hightemperature/short-time heating in view of the physical characteristicsof a film. In particular, a falling film is ideally suited to rapidheating of a product because it is by nature a thin distribution of theproduct with a high ratio of heat transfer surface area to volume andoptimum heat transfer characteristics. Thus all particles of the filmcan be rapidly heated to the desired temperature with excellentuniformity. The fact that the film is a continuous body of liquidenhances the uniformity of heating relative to a spray or otherarrangement where droplets are separated and travel for different timesalong various trajectories which cannot be fully controlled. The fallingfilm concept was also ideally suited to short heating times because theexposure time of the product to the heated environment can be veryaccurately controlled simply by controlling the height of the fallingfilm. For these reasons Davies and Petersen selected a falling film asthe product configuration optimally suited to the optimaltime-temperature configuration they had experimentally determined forminimizing the denaturation of proteins in sterilized milk.Unfortunately, the successful formation and continuous maintenance of afalling film proved to be an extremely difficult technical problem whichDavies and Petersen were unable to solve. The disclosure in theabove-referenced Davies et al patent sets forth their proposed techniqueof providing a film which adheres by surface tension to guide plates,and is heated while in contact with these guide plates. For reasonswhich are made clear elsewhere in the present specification, heating afalling film while it is in contact with a guide plate of this nature isnot suitable from a practical standpoint because flavor distortionoccurs and the product burns onto the guide plate after a short periodof use. Nevertheless the discovery that a falling film of product isideally suited to the time-temperature relationship developed in theDavies et al patent remains an important advance in the state of the artof milk sterilization.

Of the prior art devices, the most advanced for producing sterilizedmilk that maintains taste qualities similar to that of fresh milk isdisclosed in U.S. Pat. No. 3,771,434 to Davies, issued Nov. 13, 1973.The present invention is an improvement and an outgrowth of theapparatus disclosed and claimed in that patent. The apparatus disclosedin Davies relies upon a falling film of liquid milk which is guided bycontact with a length of screen, wherein the falling film is subjectedto high temperature steam for a short interval to cause sterilization. Anumber of important refinements have now been discovered whichsubstantially improve its performance. More specifically,experimentation with the system disclosed in Davies has revealed thatproduct taste, quality and long-term consistency could be significantlyimproved with proper modification of the disclosed system. It should benoted that the device disclosed in the Davies patent is far differentfrom devices which have been relied upon in the past for evaporation ofliquids. A device used for evaporation is disclosed, for example, in theMonsanto U.S. Pat. No. 441,106 issued on Nov. 18, 1890. In that patent aliquid is divided into fine droplets and subjected to heating wherebyrapid evaporation of the falling liquid droplets occurs. Naturally, theuse of such a system would be disastrous in the production of sterilizedliquid milk because the evaporation which would occur, even if it wereonly partial, would significantly change the consistency of the milk,thereby making it highly undesirable to consumers.

A need therefore exists for an improved sterilization system for fluidor liquid foods wherein complete sterilization is obtained withoutadversely effecting the taste or other qualities of the food product.

SUMMARY OF THE INVENTION

Accordingly, one object of this invention is to provide an improvedprocess and apparatus for sterilizing food products without adverselyeffecting their taste or other physical properties.

Another object of the present invention is the provision of a novelmethod and apparatus for heating fluids to a selected temperature for aselected interval of time with a minimum of turbulence, agitation orphysical stress.

Yet another object of the present invention is the provision of a novelmethod and apparatus for sterilizing fluid foods, such as milk with aminimum of thermal and physical perturbation.

Another object of the present invention is the provision of a novelmethod and apparatus for sterilizing milk which causes the leastpossible denaturation of whey proteins and comparable to that ofpasteurization.

A still further object of the present invention is the provision of anovel method and apparatus for sterilizing fluid foods wherein anisolated film of the product is formed and is subjected to heattreatment at a particular temperature for a selected time interval,during which interval it is subjected to an absolute minimum of physicalstress.

Briefly, these and other objects of the invention are attained by theprovision of a unique apparatus in which the fluid to be treated isfirst formed into a continuous isolated film which is kept free fromcontact with any surface hotter than itself for a selected interval oftime. The apparatus is designed so that the film remains intact as acontinuous film for the entire period during which it is subjected toheat treatment. Heat treatment is provided by subjecting the film to ahigh temperature gas, such as steam, which is supplied so as to preventany turbulence which would disturb the fluid film. The film is rapidlycooled subsequent to heat treatment. Details of the apparatus utilizedin this process are also disclosed, as is the control system for theoverall process and apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration on one form of prior art sterilizingsystem, from Davies et al U.S. Pat. No. 2,899,320;

FIG. 2 is a cut-away illustration of another form of prior artsterilizing system from Evans U.S. Pat. No. 3,032,423;

FIG. 3 is a cut-away illustration showing further details of the priorart apparatus shown in FIG. 2;

FIG. 4 is a cut-away illustration showing an alternative embodiment ofthe prior art system shown in FIG. 3;

FIG. 5 is a perspective illustration of a film forming head and isolatedfilm in accordance with the present invention;

FIG. 6 is a cut-away side view of the film forming head shown in FIG. 5;

FIG. 7 is a plan view of a distribution plate;

FIG. 8 is a perspective illustration of a Teflon sheet covering for adistribution plate;

FIG. 9 is a perspective illustration of a film-forming head modified toinclude the structure shown in FIG. 8;

FIG. 10 is a side view of an end cap structure for a film forming head;

FIG. 11 is an illustration of an end feed film forming head;

FIG. 12 is a perspective illustration of a flow distribution tube foruse in the structure of FIG. 11;

FIG. 13 is a cut-away end view of the end feed film forming head shownin FIG. 11;

FIG. 14A is a side view of a film forming head;

FIG. 14B is a cut-away end view of the structure shown in FIG. 14A;

FIG. 15 is a perspective illustration of a two-film branching network;

FIG. 16 is a perspective illustration of a four-film branching network;

FIG. 17 is a top plan view of a radial multiple-film forming head;

FIG. 18 is a top plan view of a concentric multiple-film forming head;

FIG. 19 is a top plan view of a parallel multiple-film forming head;

FIG. 20 is a cut away side view of a sterilizaton chamber or ultra hightemperature (UHT) heater in accordance with the teachings of the presentinvention;

FIG. 21 is a cut away partially schematic view of the structure shown inFIG. 20 illustrating steam flow therein;

FIG. 22 is a top plan view of a steam distribution plate;

FIG. 23 is a schematic diagram illustrating input and output couplingsto the sterilization chamber of the invention;

FIG. 24 is a schematic illustration in perspective of a liquidprocessing system employing the sterilization chamber of the invention,and;

FIG. 25 is a perspective illustration of an alternative steamdistribution device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the present invention is applicable to an unlimited variety offluent or liquid products, including such products as beer, orangejuice, soup containing particulate matter such as meat and vegetablesand other non-food products, many of the detailed aspects of thepreferred embodiments are described as utilized with milk, since of allfoods milk is perhaps the most complex and requires the most delicateand precise handling in its sterilizaton if flavor distortion is to beprevented. Accordingly emphasis is placed in this specification on thetreatment of milk with the understanding that numerous other foods canbe treated in substantially the same manner but with much lesscomplexity.

Attention is first directed to TABLE 1 which deals with thesterilization of milk. In this TABLE a number of thermal and physicaleffects are described in the left-hand column while the resultantdistortions to flavor or other physical properties of the milk are setforth in the right-hand column. TABLE 1 points out the uniquesensitivity of milk to heat treatment, and particularly emphasizes thefact that milk is especially sensitive to heat treatment (thermalperturbation) and to physical perturbations (i.e. excessive agitation)during heating. A technique for eliminating the undesirable effects ofthermal perturbations in milk sterilization is set forth in theabove-referenced Davies et al U.S. Pat. No. 2,899,320.

                                      TABLE 1                                     __________________________________________________________________________                            Resulting Distortion In                                                       Flavor Or Other Physical                                     Thermal/Physical Perturbation                                                                  Properties of Milk                                    __________________________________________________________________________           Heating of Lactose and Casein                                                                  Browning                                                     Together In Milk                                                              Maintaining Milk at a Temperature                                                              Albumin Content Starts                                       above 165° for More Than 30                                                             to Coagulate and Sulf-                                       Seconds          hydryls Form From One                                                         or More Proteins Present                                                      in Milk, Particularly                                 Thermal                 Beta Lactoglobulin Pro-                                                       tein, Sulfhydryls Releas-                                                     ed.                                                          General Exposure to Tempera-                                                                   Coagulation, Increase in                                     tures Above 165°                                                                        Viscosity and "Off" Fla-                                                      vor, Release of Sulfhy-                                                       dryls Causing Cooked                                                          Flavor.                                                      Agitation During Heating                                                                       Coagulation Occurs More                                                       Readily                                                      Exposure of Milk to Metal and                                                                  Burned or Scorched                                           Other Surfaces at Significantly                                                                Flavor                                                       Higher Temperature Than the                                                   Milk                                                                          Turbulence, Agitation and Phy-                                                                 Sandy, Chalky Taste or                                Physical                                                                             sical Stress at High Temperatures                                                              Coconut Flavor Results                                       Improper Steam Injection                                                                       Sandy taste, Sediments,                                                       Oiling Off                                                   Agitation and Turbulence in                                                                    Oiling Off, Fat Separa-                                      Holding Tube     tion                                                         Changing Parameters Anywhere                                                                   Inconsistency in Product                                     in Sterilizing System                                                                          Quality or Taste                                      __________________________________________________________________________

In that patent it is revealed that a proper time-temperaturerelationship is one of the keys to removing the undesirable effects ofthermal perturbation. In particular, if milk is heated to a maximumtemperature of 300° F. for a period of between 1.5 and 3.0 seconds, theappropriate heat treatment for sterilization is attained without thermaltaste distortion. However, extensive research based upon the inventiondisclosed in the Davies et al patent has revealed that adherence to theteachings of that patent alone are not sufficient to produce milk whichis free of flavor distortion. It has been discovered that milk isextremely sensitive to physical perturbations while subject to thermalstress. In other words, any substantial turbulence, agitation orphysical stress experienced by the milk while it is at the hightemperature required for sterilization causes an unmistakable change inthe flavor of the milk. Specifically, the milk may develop a chalky orsandy taste or flavor. Similarly, a scorched or burned flavor maydevelop if the milk engages a surface which is hotter than that of themilk itself even though contact may occur over a small surface and for ashort time, such as over plates and screens. Accordingly, a problemthought by many to be insurmountable was presented to the presentinventors: how to heat milk to a temperature of approximately 300° for aperiod of only one second and then rapidly reduce the temperature below165° while preventing the milk from experiencing any substantialturbulence, agitation or physical stress and preventing the milk fromengaging a surface having a higher temperature than the milk itself.Furthermore, the solution, to have any commercial merit, required asystem which was relatively simple to construct, inexpensive to produce,and self-cleaning to the maximum extent possible so that it could easilybe maintained in sterile condition for long production runs. Asuccessful device capable of commercial exploitation, must solve all ofthese problems and maintain a fully uniform or consistent outputproduct.

Although the previously mentioned patent to Davies (U.S. Pat. No.3,771,434) provided the closest approach to the solution of this problemknown at the time that application was filed (1972), extensive researchconducted by the Applicants has now revealed a series of importantimprovements which yield the desired result, that is, a truly sterilizedmilk which cannot be distinguished by the consumer from conventionalfresh milk. As pointed out previously, it should be noted that thepresent application is couched in terms of processing milk because ofthe unique sensitivity of milk to heat treatment. Substantially allother known liquid products, including foods and other types of liquidproducts, can also be processed according to the same technique, sincemost other products do not have the extreme sensitivity exhibited bymilk to physical stress during heating.

In view of the significant sensitivity of milk to physical stress duringheating, a significant aspect of the present invention is the provisionof a unique method and apparatus for physically handling the milk duringthe process of heating it. This technique has been arrived at aftersubstantial research and permits the milk to be heated with the leastamount of physical stress, turbulence or agitation. It further permitsthe milk to be heated without coming into contact with any surfacehotter than the milk itself. These two factors are significant, but maybe subject to misinterpretation in view of what has been done in theprior art. Accordingly a brief summary of exemplary prior art in thisarea is considered to be necessary to fully understand the uniquedevelopments of the present invention.

As previously explained, a falling "film" of milk is ideally suited tothe high temperature/short interval heating process required to producesterilized milk of good flavor quality. Examples of falling films aredisclosed in the Davies et al U.S. Pat. No. 2,899,320 (see FIG. 1) andthe Evans U.S. Pat. No. 3,032,423 (see FIGS. 2, 3 and 4). While each ofthese patents discloses a falling film of milk in a heating vessel, thefalling film is not isolated in space, but is held by surface tension tovertically disposed plates (designated 5 in Davies et al and 35 inEvans) for guiding the milk through a heating chamber. It was originallythought that the use of such plates would lead to uniformity in heatingthe milk since the plates would be maintained at a relatively high andconstant temperature by steam or some other heated medium circulatingwithin the heating vessel. However, it has been discovered that theexposure to such guide plates at high temperature causes product burn-onand adversely effects the taste of the resulting milk product. Inparticular, the taste of the milk is adversely effected by exposure to ametal surface during heating and also by exposure to a surface which ishotter than the milk itself. In the case of the vertical guide platesmentioned, it was not realized (see Davies et al, Col. 6, lines 40-45)that these surfaces become hotter than the milk being fed into theapparatus. Specifically, it has been experimentally discovered thatguide plates of the type used in Davies et al and Evans (see FIG. 3)overheat at certain spots even when covered with the flowing product.Resultant "hot spots" cause the flowing product to burn onto the plateor screen. Once "burn-on" starts it rapidly gets worse and causesundesirable buildups of burned product to grow quickly, causing flavordistortion and disruption of the product flow which soon destroysuniformity of the falling film of product. "Hot spots" commonly occur atedges, weld spots, etc., and it is virtually impossible to completelyeliminate them.

The same effect is illustrated in FIG. 4. In this case a screen 39 isused to form the film, but "hot spots" and burn-on continue to occur. Asimilar arrangement using a screen is illustrated in Davies U.S. Pat.No. 3,771,434, previously mentioned, and a similar effect occurs there.In addition, the problem of maintaining the screen sanitary is asignificant one. As the screen has many small openings in its mesh, fineparticles of material invariably collect on the screen surface. Thesematerials are extremely difficult to dislodge during any cleaningperiod, and accordingly it is difficult to maintain the equipment in asterile and fully sanitary condition after a short period of operation.Furthermore, clogging of the screen destroys the falling film and causesthe device to stop operating effectively after a short time. Accordinglythe presence of a screen in such an apparatus can three separateproblems, taste distortion, accumulation of particles leading to thelack of a sterile environment, and breaking and distortions of thefalling film.

In contrast to the prior art apparatuses described above, the presentapparatus, illustrated in FIG. 5, includes a supply pipe 52 feeding afilm forming head 54 comprised of a cylindrical length of pipe with aslit 56 along a lower surface thereof. The purpose of the film forminghead is to form a thin, continuous isolated film, designated 58 in thedrawings. Since the formation of the film is of considerable importanceto the operation of the present invention, further details of the natureand formation of the film will be presented.

It is first pointed out that the isolated film of the present inventionis a continuous film. By continuous is meant that the film is neverbroken into droplets, nor is any portion every disconnected from thecentral body of the film in the course of its fall and heating in thesterilizing apparatus of the present invention. This is in directcontrast to certain devices disclosed in the prior art which have beenused particularly for the purpose of drying or evaporating liquids. Forexample, attention is directed to the Monsanto patent (U.S. Pat. No.441,106 issued Nov. 18, 1890) and the Okada patent (U.S. Pat. No.3,621,902 issued Nov. 23, 1971). In these patents the liquid material tobe processed is sprayed or dropped from an appropriate distributionmanifold into a heated atmosphere. However, the purpose of the sprayingor dropping is to create finely divided particles or droplets of thematerial which provide a large surface area to permit rapid evaporationof water within the material being processed to speed evaporation.Evaporation of this sort would, of course, totally destroy the naturalquality of milk sought for in accordance with the teachings of thepresent invention. In contrast, the present invention deals exclusivelywith a falling continuous film of milk from which every particle isconnected to every other and no isolated droplets are formed.

The film 58 is also isolated because, once it leaves the slit 56, itnever engages anything until it reaches the bottom of the sterilizingchamber (to be described subsequently). Because it is thus isolated fromall components within the sterilizing chamber, the film is notcontaminated by engaging any surface hotter than itself in contrast totypical prior art devices shown in FIGS. 1-4.

Because of the important function that the thin continuous isolated filmhas in the context of the present invention, a considerable amount ofattention has been directed by the Inventors to the proper forming ofthis film and to forming the film in such a way that the film continuesto be formed without interruption during lengthy processing runs in theapparatus of the invention. It has been observed experimentally that theconfiguration and appearance of a free-falling film or column of liquidchanges considerably depending upon the initial velocity of the liquidprior to free fall. If the velocity is too high, droplets of liquidform, some spraying or splashing occurs and the surface of the fallingbody of liquid is not smooth. If the velocity or flow rate of the liquidis within a specified range, however, the falling body of liquid forms acontinuous unbroken surface in free fall with a mirror-like surface andno splashing or spraying of particles results, even when the fallingbody of liquid impinges on a rigid surface. If the velocity or flow rateis too low, then the continuous body of falling liquid breaks intodroplets since the amount of liquid in free fall is not sufficient tomaintain the continuous surface of the film or column. Again sprayingand splashing of liquid particles occurs.

Experimental measurements on free-falling liquid bodies passing througha slit indicate that liquids must have an average initial flow velocityfalling within a prescribed range to form a continuous free fallingbody. In the case of water, for example, the following measurementsapply:

For an initial velocity of less than approximately 1.5 feet per second,the falling film or column breaks up and the flow is not sufficient tomaintain the falling liquid in a continuous state. For initialvelocities between 1.5 and 3.5 ft. per second, the falling bodymaintains a smooth and perfectly continuous surface. For initialvelocities above 3.5 ft. per second, some splashing occurs and thesurface of the falling body of liquid is no longer smooth andcontinuous.

This experimental data indicates that the existence of a relationshipbetween the surface tension forces acting on the particles of fallingliquid, the forces of motion created by the initial velocity of theliquid prior to its entering a state of free fall, and the gravitationalforces acting on the liquid during free fall. For water with an initialvelocity between 1.5 and 3.5 ft. per second, an equilibrium condition isreached among these various forces resulting in the formation of acontinuous laminar film or column of free falling liquid. Thisequilibrium may be destroyed if one or more of the variable forces issignificantly changed. For example, a liquid with a vescosity differentfrom that of water has a different range of initial velocities if acontinuous free falling body of liquid is to be formed. However, allliquids, no matter what their viscosity, can be made to form continuousbody in free fall simply by measuring the appropriate parameters of theliquid and appropriately controlling the forces acting on the liquid.

In the case of the present invention, where the liquid falls into asteam heated pressure vessel, the effect of steam flow on the fallingliquid must also be considered. If the velocity of the steam acting onthe liquid is sufficiently high, the falling body of water will bebroken up and the formation of a continuous falling body will becomeimpossible. Similarly, if the fall body of liquid is exceptionally thinand fragile, then even small stream currents will cause it to break upresulting in spraying and splashing of the falling liquid.

Based on these considerations, the following method for designing a freefalling film of liquid passing through a slit has been developed inaccordance with the concepts of the present invention:

1. The liquid is first studied to determine the optimum initial velocityat which a continuous body of the liquid will be formed and maintainedwhen the liquid is in free fall.

2. The desired flow rate of the falling film is determined.

3. The length of the slit through which the film must fall is determinedfrom the dimensions of the vessel in which it is placed, or othersimilar physical constraints.

4. The width of the slit is calculated so as to provide an initialvelocity which is the optimum velocity of the liquid for forming acontinuous body while in free fall at the determined flow rate of theliquid. Either the length or width of the slit, or both, can be variedto arrive at the desired cross sectional area which produces the desiredaverage initial velocity.

5. The liquid flow must be evenly distributed throughout the slit sothat approximately the same volume of liquid falls through each linearsegment of the slit.

The last of these factors, evenly distributing the liquid flowthroughout the slit, has created a serious technical problem. If, forexample, an arrangement of the type illustrated in FIG. 5 is usedwithout any internal flow distribution structure, a continuous filmcannot be formed. This is because fluid supplied through the input pipe52 will not inherently distribute itself uniformly throughout the lengthof the film forming head 54. In fact, a distinct pressure minimum willbe positioned at the center of the flow distribution head while pressuremaxima will exist at opposite ends thereof. Accordingly a flowdistribution structure of the type illustrated in FIG. 6 is required.

As shown in FIG. 6, the supply pipe 52 feeds the film forming head 54.The film forming head itself is constructed of an inner pipe 60 which isdivided into an upper chamber 62 and a lower chamber 64 by adistribution plate 66. The upper chamber 62 is in direct communicationwith the supply pipe 52 and is always completely filled with liquid.Flow rates and other parameters are determined so that the liquidsurface indicated by the line 68 entirely covers the distribution plate66 at all times. The distribution plate contains a plurality ofdistribution apertures 70, as illustrated in FIG. 7. The distributionapertures may be sized progressively with the largest at the center andthe smallest near the extremities of the distribution plate. Thus inthis arrangement the smallest distribution apertures 70 are found nearthe ends of the film forming head 54 where the fluid pressure isnormally highest, while the largest distribution apertures are near thecenter of the film forming head where the pressure is normally lowest.The purpose and function of this arrangement is to provide asubstantially uniform flow through the distribution plate 66 throughoutits length. The actual sizing of the holes can be calculated easilyusing conventional mathematical analysis. Furthermore, as an alternativeto varying the size of the holes along the length of the distributionplate, small holes of equal size can be used with the distribution ofholes arranged to provide uniformity of flow through the distributionplate along its entire length.

The distribution plate is thus used to solve the problem of producinguniform output flow along the entire length of the flow-forming head 54.However, the individual streams of liquid falling through the apertures70 in the distribution plate must be integrated to form a continuousfilm. For this reason, a trough 72 is supported within the lowestchamber 64 of the inner pipe 60 to collect the liquid falling throughthe distribution apertures 70. The trough 72 is supported within thelower chamber 64 by means of suitable mounting members 74 which arearranged so as not to impede the flow of liquid out of the trough overthe top of its side walls 76 and out through the slit 56. This combinedstructure thus serves several purposes. First, flow is evenlydistributed throughout the length of the film forming head by thedistribution plate 66. Second, the liquid dropping through thedistribution plate is integrated into a continuous body by the trough72. Third, the trough is permitted to overflow so that the liquid flowsalong the walls of the lower chamber 64 of the inner pipe 60 and outslit 58 in the form of a continuous, isolated free-falling film 58.

The apparatus of FIG. 6 is normally constructed of stainless steel withexternal insulation 75 being formed of a suitable material resistant todegradation at high temperatures. With this structure, both sides of theslit 56 are formed of stainless steel since the inner pipe 60 is formedof that material. In some instances, for example with such products asice cream mixes which contain sugar, stabilizers, emulsifiers and thelike, some burning of the product onto the stainless steel edges of theslit 56 can occur. Such burning of the product can develop depositswhich may stop flow through the slit or otherwise disturb thefree-falling film 58. To eliminate this problem, a modified film forminghead was developed, as illustrated in FIGS. 8 and 9.

As shown in FIG. 8, a sheet of Teflon or equivalent material 78 isdraped over the distribution plate 66 and held in place on thedistribution plate by means of appropriate stainless steel wires 80.Apertures are formed in the Teflon sheet 78 to correspond to thedistribution apertures 70 in the distribution plate. The Teflon sheet 78is substantially wider than the distribution plate 66 thereby formingflexible side curtains 82 on opposite sides of the distribution plate.The distribution plate with the Teflon sheet draped over it is insertedinto the inner pipe 60, as shown in FIG. 9. The Teflon side curtains 82initially conform generally to the configuration of the inner surface ofthe pipe 60 and thus prevent the fluid dropping through the distributionapertures 70 from contacting the interior surface of the pipe 60.Furthermore, a portion of each side curtain 82 extends through the slit56 forming a pair of flexible lips 84 hanging slightly below the filmforming head 54. The thin Teflon sheet 78 thus provides both an inertsurface separating the liquid product from the stainless steel surfaceof the inner pipe 60 (and the stainless steel sides of the slit 56) andfurther provides a flexible guide for forming the falling film 58. Thisarrangement provides a number of advantages over the structureillustrated in FIGS. 6 and 7. First, the Teflon guide eliminates contactbetween the product and the metal of slit 58, and thus prevents productburn-on. Furthermore, it renders the film forming head more easilycleanable in place, and thus more highly sanitary than the previouslydescribed structure. The Teflon sheet 78 also acts as a sealing gasketbetween the distribution plate 66 and the surface of the inner pipe 60.The flexibility of the Teflon sheet 78 and the curvature of the insidewalls of the pipe 60 permit the side curtains 82 of the Teflon sheets toform a naturally narrowing channel. Furthermore, the Teflon sheets willtend to be attracted by surface tension to the fluid flowing through theslit 56, and thus will tend to adhere to the body of the fluid containedin the lower chamber 64 of the film forming head. As a result, theTeflon sheet 78 eliminates the need for the trough 72 in the embodimentillustrated in FIG. 6. In forming the falling film 58, tension in theTeflon side curtains 82 and flexible lips 84 will keep the Teflon lips84 pressed against each other. The product pressure from inside the filmforming head will force the lips 84 apart, thereby permitting theproduct to drop through the slit 56 while at the same time forming theproduct into the isolated, continuous falling film 58. This feature ofthe embodiment of the invention illustrated in FIGS. 8 and 9 isimportant since it allows a large variation in flow rates as well as theuse of the invention in processing particulate matter. It also preventssteam in the sterilization chamber, described subsequently, fromentering the distribution tube. This structure is also easilyadjustable, extremely inexpensive and extraordinarily easy to installand remove for cleaning or repair.

The open ends of the film forming head 54 are preferably closed by aremovable end cap 86 as shown in FIG. 10. The end cap includes astainless steel cap 88 having an extended rim 90 having an innerdiameter somewhat greater than the outer diameter of the inner pipe 60so that the rim fits around the pipe 60. Within the cap 88 is a sealinggasket 92 formed of Teflon, suitable rubber or another appropriatematerial of similar properties. The gasket presses against the open endsurface, designated 94, of the inner pipe 60 providing a fluid tightseal.

The end cap 86 is held in place by a suitable overcenter or snap-actionmechanism 96. This mechanism includes a pair of bearing members 98 (onlyone shown) secured to opposite sides of the outer surface of pipe 60.These bearing surfaces have a central recess 100 into which each of thetwo legs 102 of the actuator arm 104 are pivotally inserted. Theactuator arm includes a pair of circular coupling points 106, one ofwhich is located on either side of the pipe 60. A coupling member 108 ofgeneral U-shaped configuration is connected to each of the circularcoupling points 106 on opposite sides of the pipe 60 and extends acrossa groove 110 in the end surface of cap 88.

By appropriate motion of the actuator arm 104, the end cap is eitherpressed in position to seal the open end of the inner gap 60 or isreleased from the pipe 60 to permit easy access to the interior thereof.A similar cap is positioned at the opposite end of the pipe 60.

The film forming head shown in FIGS. 6-10 is characterized as a "centerfeed" head because the fluid supplied to the head is delivered throughthe inlet pipe 52 which is positioned at the center of the film forminghead 54. An alternative to this structure is the "end feed head"illustrated in FIG. 11. In this arrangement, the supply pipe 52 iscoupled to a flow dividing manifold 112 which is coupled to both ends ofa modified film forming head 114, whereby an equal flow of liquid issupplied to opposite ends of the head 114. This design has severalimportant advantages over the center feed design previously described.First, no "dead flow" spots are created in the end feed design, thusproviding a potentially more sanitary arrangement. The flow distributorcan also be more easily cleaned and serviced in place. Furthermore, theflow is more evenly distributed and splitting or other deformation ofthe falling film 58 is less likely to occur. In addition the need forthe end cap structure illustrated in FIG. 10 is eliminated.

The modified film forming head 114 retains the slit 56 for forming thefilm 58 and the entire assembly is fully insulated, although theinsulation is not shown in FIG. 11. However, the film distribution plate66 is eliminated and replaced by a flow distribution tube 116. The flowdistribution tube is inserted into a length of stainless steel pipe 118having the slit 56 in a lower surface thereof, and appropriate mountingflanges 120 and 122 to permit a fluid tight coupling with the flowdividing manifold 112.

The flow distribution tube 116 is illustrated in more detail in FIG. 12as including a central body 124 having a conical end flange 126 ateither end thereof. The end flange 126 may be either formed integrallywith the central body 124 or may be coupled to the central body byconventional means. The outer diameter of the central body 124 is lessthen the inner diameter of the pipe 118 to provide a flow space 128between these two structures. The end flange 126 at either end of theflow distribution tube is expanded to the same diameter as the innerdiameter of the pipe 118, thereby providing a fluid seal for directingall fluid from the flow dividing manifold 112 into the interior openingof the flow distribution tube 116. Fluid within the flow distributiontube escapes through distribution apertures 130 drilled through thecentral body 124 of the distribution tube. These apertures are varied insize or density in the same manner as previously described with respectto the distribution aperture 70 of the distribution plate 66. However,the distribution pattern is different in the case of the apparatusillustrated in FIG. 12 because the pressure distribution in the end feedsystem is different from that in the center feed system, as will beapparent to those skilled in the art. Specifically, the size or densityof the holes should be decreased toward the center of the flowdistribution tube 116 and increased toward the ends thereof. The flowdistribution apertures 130 are oriented toward the top surface of thedistribution tube when the distribution tube is in place in thedistribution head. When assembled in this manner, fluid flowing throughthe interior of the distribution tube 116 passes out through thedistribution apertures 130 and flows around the outer surface of thedistribution tube into the flow space 128 and out through the slot 56 toform the continuous isolated film 58.

A Teflon sheet 132 can be used with the end feed design shown in FIG. 11in somewhat the same manner as the Teflon sheet 78 is used with thecenter feed system. As shown in FIG. 13, the Teflon sheet 132 is wrappedaround the distribution tube 116 the outer surface of the flowdistribution tube 116, but is maintained at the same diameter as theflanges, and is not closely pressed against the distribution apertures130. In other words, the flow space 128 continues to exist between theflow distribution tube 116 and the Teflon sheet 132. Accordingly thereis no need to drill holes in the Teflon sheet. The Teflon sheet passesthrough the slit 56 and again provides flexible lips 84 through whichthe liquid passes to form the thin, continuous isolated film 58.

It is noted that the film forming heads of FIGS. 7 and 11 are presentlyillustrated as being formed fundamentally of stainless steel. The samestructures can also be formed of solid Teflon, with appropriateapertures being drilled or formed in the Teflon. However, in this casethe apertures must be stablized by appropriate rigid inserts. Forexample, the slit 56 in a solid Teflon block must be stabilized by steelor an equivalent rigid structure, as it will contract and close whensubjected to high temperatures if it is not appropriately stabilized.

In many instances it is desirable to have more than one film forminghead in use at the same time. For instance, it may be desirable to havea plurality of heads operating within a sterilization device in order toincrease the flow handling capability of the device and efficientlyutilize the interior space of the device. In the past, a manual valvehas been utilized for controlling the flow to each film forming head.However, when several films are in use it becomes difficult to properlyadjust the valves to provide appropriate flow distribution to all filmforming heads. As a result, unequal flow distribution sometimes occurscausing one or more films to be imperfectly formed so that splashing canoccur causing droplets to burn onto hot surfaces of the sterilizationdevice causing some contamination of the treated product. Furthermore,unless a flowmeter is used in each of the feed pipes, it becomesdifficult for an operator to adjust the valves and visually determinethe appropriate film formation for each falling film. To eliminate theexpense of multiple flowmeters and other disadvantages, a uniqueapproach to solving this fluid flow distribution problem has beendeveloped and will now be described mathematically.

The flow equation for an incompressible liquid flowing through anorifice in a pipe is (see Mechanics of Fluids; Shames, Irving H., McGrawHill, 1962, p. 168); ##EQU1## Where: Cd is the coefficient of friction

Q is the flow rate

A₁ is the cross sectional area of the pipe

A₂ is the cross sectional area of the orifice (slit)

P₁ is the pressure before the orifice (slit)

P₂ is the pressure after the orifice (slit)

ρ is the density of the liquid

For A₁ >>A₂ the flow can be approximated as: ##EQU2## Equation 2 revealsthat the flow in a pipe and through a slit orifice is directlyproportional to the cross sectional area of the slit and directlyproportional to the square root of the pressure.

FIGS. 14A and 14B depict a film forming head with an inner pipe 60having a slit 56. According to equation 2, the flow through the slitwill depend on the square root of P1-P2 and cross sectional area of theslit which is the length of the pipe times the slit width (Lt). If thedifference between P1 and P2 remains constant or P1 remains constant,(since P2 is constant for all tubes), then the flow Q will beproportional to the area of the slit opening. If the slit width (t) isalso the same for all tubes, then the flow Q will be proportional to thelength of the tube.

For more than one film forming head in a sterilizer, if the pressure inthe vertical pipe is the same for all heads, then the flow Q would bedistributed proportional to the length of the film, thus providingoptimum film formation for all films at a specified flow rate. Thedesign for distributing the flow coming into the sterilizer should besuch that the pressure in the film forming heads is equal for all films.If the slit widths are the same for all film forming heads, then theflow will be directly proportional to the slit length.

A very simple design incorporating these concepts is shown in FIG. 15for two films and in FIG. 16 for four films. The two film designillustrated in FIG. 14 includes the flow supply pipe 52 feeding a branchpipe 134 having two arms, each coupled to a film forming head 140, 142.These film forming heads may be equivalent in design to either thecenter feed or end feed units previously described. The diameter of thebranch pipe 134 is constant and is the same in both arms 136 and 138. Ifone of the film forming heads is shorter than the other, then accordingto the equations set forth above the flow will be unequally distributedin the arms 136 and 138, the arm leading to the film forming head withthe longer slit receiving the greater quantity of fluid. Similarly, inFIG. 16 a distribution unit for forming four separate films isillustrated. In this unit a supply pipe 52 is coupled to branch pipe 134again having arms 136 and 138. However, the arm 136 is coupled to asecond branch pipe 144 having arms 146 and 148 which are coupled to filmforming heads 150 and 152. Similarly the branch pipe arm 138 is coupledto a second branch pipe 154 which in turn has two arms 156 and 158coupled respectively to film forming heads 160 and 162. Thus thearrangement in FIG. 16 is similar to that in FIG. 15, but with anadditional stage of distribution added. The same system can be scaled upto handle essentially any number of film forming heads. Although shownonly for an even number of heads, the system works equally well for anodd number of heads. However, some orificing of the flow may benecessary.

Using this design, equal pressure would be provided for all film formingheads. The distributing system does not need any valves or any operatorinteraction. It is flexible in that unneeded film supply pipes may becapped off, and it provides optimum film formation for a given flow.

Although the film forming heads have been illustrated as being formed ofstraight or linear lengths of pipe, or of linear pipes arranged inparallel, they need not be limited to these types of configurations.Other configurations for the film forming heads work equally well. Forexample, attention is directed to FIG. 17 showing a film forming head164 comprised of a plurality of film forming tubes 166 extendingradially from a central feed pipe 52. This arrangement produces aplurality of angularly separated isolated, continuous falling filmswhich do not interfere with one another as they fall.

Another arrangement is illustrated in FIG. 18 wherein a plurality ofconcentric film falling heads 168 are shown. These heads produceisolated, continuous falling films in the form of concentric fallingcurtains.

Yet another film forming head 170 is illustrated in FIG. 19 as having aplurality of film forming tubes 172 arranged in parallel and all coupledto a distribution manifold 174. Similarly, other alternative structuresmay also be used, as will be apparent to those skilled in the art.

Referring now to FIG. 20, the film 58 of the present invention is shownpositioned within a sterilization chamber or UHT unit 176. Thesterilization chamber 176 is based on that disclosed in Davies U.S. Pat.No. 3,771,434 in terms of its general structural configuration includingits insulated outer wall structure 177 and conical lower portion 180.The film forming head 54 including slit 56 is an improvement inaccordance with the present invention, however. It should be noted inthis regard that a plurality of film forming heads of any of thepreviously disclosed types and configurations can be used in thestructure illustrated in FIG. 20 as can the flow distribution devices ofFIGS. 15 and 16.

Referring again to FIG. 20, the sterilization chamber 176 includes asteam inlet manifold 178 to permit entry of the high temperature steamin such a way that sterilization of the falling continuous, isolatedfilm 58 can take place. The lower portion of the sterilization chamber176 is formed into conical section 180 upon which the fallingcontinuous, isolated film 58 impinges and is collected for withdrawalthrough an outlet pipe 182. The interior surface of the conical section180 may be coated with a suitable inert plastic material such as Teflon,as indicated at 184. The conical section 180 may also be provided with acooling jacket 186 to which a cooling fluid such as air or water issupplied through a pair of inlet pipes 188 and is withdrawn through apair of outlet pipes 190. The use of the Teflon coating 184 is importantin that it prevents the heated falling continuous, isolated film fromengaging a metal surface while at a high temperature within thesterilization chamber, to thereby prevent any possible flavor distortioncaused by contact of the hot milk with a metal surface. The coolingjacket 186 cools the conical section 180 as well as the inner Tefloncoating to a temperature below the temperature of the falling column ofmilk 58 (e.g. about 280° F.). Thus the falling column of milk impingesupon a surface 184 which is cooler than itself, so that the earliermentioned criterion is met: the heated milk never impinges upon asurface which is hotter than (or even as hot as) itself in the course ofthe sterilizing process. This has been experimentally determined to be ahighly significant factor in maintaining proper flavor quality in thesterilized milk.

It was experimentally found that using water as the coolant andincreasing the flow rate of water through the jacket and thus thecooling of the cone, decreases the amount of burn-on. When the wateroutlet temperature reaches approximately 100° F., burn-on is completelyeliminated. Because of the small surface area, actual heat transfer issmall and is less than 100,000 BTU's per hour for a 12,000 qts. per hoursystem.

As an alternative to the Teflon coating 184 and cooling jacket 186, aTeflon cone 192 (shown in phantom) may be mounted within the conicalsection of the sterilization chamber upon a suitable rack 194. The coneis spaced from the walls of the conical section and has a sufficientlywide top opening to receive all of the falling liquid product. Althoughthe Teflon cone does not provide a cooling function, it does not heat-upexcessively and prevents product burn-on.

As was pointed out previously, physical perturbation of the heated milkmust be minimized to prevent flavor distortion. A good example of thetype of physical perturbation that can seriously damage the flavor ofthe milk is splashing as the film of milk 58 strikes the conical section180 of the sterilization chamber as the sterilization heating iscompleted. If the film 58 is not properly formed, and is not of theproper height, the entire film, or portions of the film adjacent theedge thereof, can become discontinuous and form droplets not connectedwith the main body of film. This is in effect a breaking of the overallunitary surface tension which holds all of the film particles together.Once droplets of this nature are formed, substantial splashing can occuras these droplets impinge upon the lower conical surface of thesterilization chamber. This splashing may cause milk particles tocontact the vertical side of the vessel, causing burn-on and flavordistortion in the portion of the milk that is highly agitated by thesplashing and this flavor distortion can contaminate the entire quantityof milk passing through the sterilizer. Accordingly it is ofconsiderable importance that the film 58 be maintained fully continuouseven after it impinges upon the cooled conical section 180 of thesterilization chamber. When the film 58 is properly regulated, it doesnot splash when engaging the conical section, thereby virtuallyeliminating severe physical perturbations or agitation from the fallingcolumn of milk. This phenomenon is analogous to, and can be demonstratedby, a falling column of water from an ordinary faucet. When the faucetis turned on slightly, drops form which fall to the sink surface andsplash, i.e. break up into very fine droplets which travel in alldirections away from the splash zone at considerable velocity. As thefaucet flow is gradually increased, the column of water becomes morecontinuous but may still break up into drops before reaching the sink ordrain surface. In this instance splashing will still occur. However,once the faucet flow reaches the proper range, a continuous stream ofwater will fall unbroken to the surface of the sink and will then spreadout evenly over a portion of that surface without any splashingoccurring whatsoever. If the water pressure is increased further,splashing will again begin to occur. These phenomena have been describedin more detail previously. The film flow in accordance with the presentinvention is set so as to be analogous to the faucet situation in whichno splashing occurs. This condition is an equilibrium condition in whichthe surface tension of the falling stream is sufficient to hold allparticles of the stream together, overcoming the disruptive (splashing)forces which occur when the stream impinges upon a surface terminatingits fall.

It should be noted that while falling columns of fluid could be used inthe context of the present invention, falling films are preferredbecause of their increased surface area which provides more rapid andmore uniform heat transfer characteristics.

In order to maintain the isolated film 58 continuous, careful formationof the film and careful control of its height for varying circumstancesmust be maintained. The isolated falling film is in fact "V" shaped inthe sense that it is narrower at the bottom than at the top. Thisshaping is caused by surface tension forces acting on the isolated filmand provides an ideal shape in that the film tends to conform to theshape of the cone at the base of the sterilizer. For a particle in themiddle of the film, the surface tension forces are equal and opposite onall sides. However, for a particle at the edge of the film, the surfacetension forces are not balanced and the particles in the film are pulledinto the film, resulting in a configuration in which the edges of thefilm become relatively thick as compared to the center of the film. Ingeneral, this problem is not critical, although it can be eliminated byfixing thin rods or wires (not shown) to the ends of the distributionhead 54 to "stretch" the film by balancing the surface tension forces.The rods are preferably formed of Teflon to minimize the chance offlavor distortion. These rods engage only a tiny portion of each edge ofthe falling film, and the film thus essentially retains its "isolated"characteristics.

The height of the continuous, isolated falling film is critical. If thefilm is too short, insufficient time will be provided for heat transferand penetration of the film, both of which are necessary for completesterilization of the material being processed. If, on the other hand,the film is too long, the film will become excessively thin because ofits continued acceleration due to the force of gravity, and will beginto break up causing splashing and other undesirable effects. Forexample, a film with an initial thickness of 0.040 inches and an initialvelocity of 2 feet per second will have the thickness to heightrelationships as shown in TABLE 2 below.

                  TABLE 2                                                         ______________________________________                                        FILM THICKNESS  HEIGHT OF FILM                                                ______________________________________                                        0.040           0 ft.                                                         0.013"          1/2 ft.                                                       0.010"          1 ft.                                                          0.0057"        2 ft.                                                          0.0044"        3 ft.                                                         ______________________________________                                    

For most products the optimum height of the falling film lies within therange between one and three feet. Films within this height range can bemaintained continuous and splash free, providing sufficient productexposure time for adequate heat treatment.

It should be noted that in the case of the present invention, thesurface upon which the falling film 58 impinges is the conical surface180 which is inclined toward the centrally positioned outlet pipe 182.The inclination of the conical surface serves to reduce the angle ofimpact of the falling film 58, and thus further reduces the likelihoodof splashing, as well as further reducing the overall physical agitationof the falling film as its direction of motion is changed by impingementupon the conical section 180. The same is true when the Teflon cone 192is used.

Summarizing the foregoing disclosure, the present invention relies on anumber of factors in supplying the fluent material to be processed tothe sterilization chamber 176. Specifically, a unique film forming headstructure is used to form a continuous fluid film. The height of thisfilm is carefully selected to maintain the film continuous and toprovide sufficient time for thorough heating to occur. The flow rate ofthe falling product is also carefully selected so that the falling filmis maintained continuous and so that no splashing occurs. Thesearrangements make possible the reliable formation and continuousexistence of an isolated film which is not guided by any mechanicalstructure, but exists independently in space for a selected interval oftime.

Another aspect of the present invention is the method and apparatus fordirectly applying heat to fluent food products or other liquids. Theconcept of direct heating means that a heated gas, such as steam,directly contacts the fluent food product material without need for anytype of mechanical or structural heat transfer mechanism. In the case ofdairy products, steam has been found to be the most efficient medium forsupplying heat to the continuous, isolated falling films previouslydisclosed. Since the heating medium is a factor of considerableimportance in attaining the intended goals of the present invention,steam parameters and flow handling are of substantial importance to theproper operation of the invention. With dairy products the steamutilized must be culinary (purified) and fully saturated with no air orother non-condensables contained in it. Such steam can be produced byconventional techniques. Furthermore, the steam must be maintained at asuitable temperature in the range of between 285° and 320° F. to permitheating of the fluent food material to the proper temperature range ofbetween 280° and 310° F. Steam pressure in the range from 40-70 psig hasalso been found to be sufficient. The rate of which steam is supplied tothe sterilization chamber 176 is also a matter of considerableimportance because the volume of steam determines the amount of heatdelivered to the sterilization chamber. Since heat is continuously beingabsorbed by the falling isolated product film, additional heat mustcontinuously be supplied in the form of more steam. It has been foundthat the approximate nominal steam flow rate for a sterilizer producing12,000 quarts per hour of product would be 3,750 pounds or 24,775 cubicfeet per hour to raise the temperature of milk from 150° F. to 300° F.Naturally a range of variation in these values is permissible, althoughthey determine an approximate optimum operating point. For sterilizingsystems of different flow rates or temperature ranges, the steam supplycan be appropriately scaled to deliver a sufficient quantity of heat tothe apparatus.

Because of the need to maintain the isolated film 58 continuous andunbroken by any form of turbulence within the sterilization chamber, thephysical handling of the applied steam is also a matter of considerableimportance. To supply a sufficient volume of steam to the apparatus, ithas been found that relatively high flow rates are often necessary, forexample on the order of 7 cubic ft. per second. Unless this flow isproperly distributed, steam velocities in excess of 100 ft. per secondwould result. Steam supplied directly to the sterilization chamber atthis velocity would, of course, totally disrupt the smooth flow of thecontinuous films within the sterilization chamber, thus rendering theinvention inoperative since the excessive turbulence produced wouldcause substantial flavor distortion for reasons already mentioned.Accordingly the steam flow within the sterilization chamber 176 must becarefully controlled.

In fact, steam must be brought in with as low a velocity as possible. Itwas found experimentally that for milk a steam velocity above 5 f.p.s.causes breaking up of the falling film. For other products the maximumsteam velocity would depend upon product viscosity and film thickness.

It is interesting to note that the maximum permisslbe steam velocityprovides a minimum size (e.g., diameter) criterion for the sterilizer176. For example, a 12,000 g.p.h. sterilizer requires 7 cubic feet ofsteam per second. If the steam is perfectly distributed, the sterilizermust have a minimum interior cross sectional area of 1.4 square feetequivalent to a diameter of 1.34 feet. A safety factor of 2 yields adiameter of approximately 2.5 feet, which has proven to be a suitablesize in practice.

In the apparatus shown in FIG. 20, saturated steam at the temperaturesand pressures previously described is applied through steam supply pipe196 to a vertical baffle 198 which surrounds the perimeter of thesterilization chamber 176. The baffle 198 is coupled to the interiorsurface of the sterilization chamber 176 at its lower end 200, and is soshaped as to provide an upwardly directed channel 202 for all steamentering through the steam supply pipe 196. Thus the velocity of theincoming steam causes it to impinge upon the adjacent surface of thevertical baffle 198 and to be subsequently directed straight upwardlythrough channel 202 along the outer wall of the sterilization chamber.Steam flow is illustrated in FIG. 21 by small arrows. As shown in thisFigure, the incoming steam flows up the steam channel 202 through theopen upper end 204 of the vertical baffle 198 and into a steamcirculation chamber 206 formed between a removable sterilization chamberlid 208 and a steam distribution plate 210, shown in more detail in FIG.22. It is noted that the lid 208 may be constructed similar to theequivalent structure shown in the previously referenced Davies patent.

The incoming steam loses much of its directional velocity in passingthrough the steam channel 202 and entering the circulation chamber 208.The steam distribution plate 210 provides the final reduction invelocity necessary to slow the steam to a non-disruptive speed and alsodistributes and directs the steam in a direction parallel to the fallingfilms to minimize its disturbing effect on the falling films 58 whilesimultaneously maximizing its absorption into the falling films. Asshown in FIGS. 21 and 22, the plate 210 is preferably circular, having adiameter which allows it to cover the entire area of the sterilizationchamber inside the vertical baffle 198. Thus the plate 210 meets theupper end 204 of the vertical baffle 198. The plate 210 is preferablysecured to lid 208 by conventional mounting members 212.

The plate 210 may be divided into two equal halves 214 and 216 to permitease of installation and removal. A plurality of feedpipe apertures 218are provided along the center line where the two halves 214 and 216 ofthe plate 210 are joined to permit the feedpipes 52 to pass through thedistribution plates 210 to supply product to the film forming heads 54.Steam distribution apertures are drilled through the distribution plate210 in rows 222. Each row has fewer holes in the center than the endssince steam pressure in the center is at a maximum, the same concept asalready described with respect to the distribution apparatus in the filmforming heads shown in FIGS. 6-13. The apertures 220 may, for example,be small holes having a typical diameter of approximately one-quarterinch and distributed in such a way as to permit a uniform but lowvelocity diffusion of steam from the circulation chamber 206 into theregion of the sterilization chamber through which the films 58 fall. Therows 222 of apertures are alligned parallel to the film forming heads54, and thus parallel to the falling films 58. Accordingly, as shown inFIG. 21, steam passing through the apertures 220 form a curtain oneither side of the falling films 58, thus providing the maximum exposureof the low velocity steam to the falling films, while at the same timereducing to an absolute minimum any disturbing influence that theflowing steam might have on the falling films. As a result almost noturbulence is experienced by the falling isolated films 58, while thefilms are rapidly heated to the desired sterilization temperature byabsorption of the steam curtains.

FIG. 25 shows an alternative to the steam flow structure illustrated inFIGS. 21 and 22 using a steam distribution system having a piping treesimilar to that shown in FIG. 15 or 16. In this instance, a first pipingtree 201 of the type illustrated in FIG. 16, for example, would be usedto handle the product and form the films 58 as shown in the figure. Asecond piping system 203 of virtually identical configuration would bepositioned adjacent the product handling system to provide steamdistribution. In other words, adjacent to each of the film forming headsin the embodiment of FIG. 16, a steam curtain forming head would bepositioned. The steam curtain head would have the same exteriorconfiguration as the illustrated film forming heads, and would simplyhave a slit or row of small apertures along the lower surface thereof topermit a steam curtain to be formed and discharged parallel to thesurfaces of the falling films. The steam pipes do not require insulationand flow characteristics are different since steam flow requires alarger cross sectional area for the slits than liquid flow.

Actual heating of the falling isolated film 58 occurs very rapidly dueto direct absorption of heated steam by the product being processed.Thus a substantial amount of heat (i.e. the heat of condensation) isreleased by the steam and transferred to the product causing a rapidtemperature increase in the product. The additional water added to theproduct by absorption of the saturated steam is subsequently removedfrom the product in the flash cooling step to be described subsequently.

It is noted that saturated steam is the suggested heating medium for usewith dairy products such as milk, in view of the need to obtain atemperature on the order of 300° F. However, other fluent products canbe treated in the system at whatever temperatures are required. In thisrespect, it is noted that the system provides a unique advantage in thatthe heat treatment temperature can be controlled to a high degree ofaccuracy heretofore not possible. Steam or other gases can be used forhigh temperatures while heated air and steam can be used at temperaturesof 200° F. and below.

Attention is now directed to FIG. 23 which is somewhat similar to FIG. 1in that it illustrates a partial system including the sterilizationchamber of sterilizer 176 coupled to a vacuum chamber 224 by means of aholding tube 226. It is noted that in operation the sterilizer 176should be placed adjacent to the vacuum chamber 224 so that a minimallength holding tube can be used. In addition, the inlet to the vacuumchamber should be at least two feet higher than the product outlet 182at the base of the sterilizer to provide a proper product flow. A sightglass 228 is shown located at approximately the center of the sterilizer176. The details of the sight glass are shown in FIG. 20 as including asmall high-intensity lamp 230, a Plexi-glass shield 232 and apressure-tight mounting structure 234. The sight glass is located topermit the operator of the system to check product flow, systemoperability, film formation, and liquid level.

Although only one steam inlet is shown in the apparatus of FIG. 20, thesystem preferably includes two steam inlets 196 positioned on oppositesides of the sterilizer 176, as illustrated in FIG. 23. The two steaminlets are fed by a common culinary steam line which delivers equalvolumes of steam to both inlets 196. This line is sized to deliverproperly filtered culinary steam to the sterilizer at an appropriatepressure, such as 75 psi.

Steam flow into the sterilizer is controlled by a conventional steamcontrol valve 236, such as a Foxboro Model V1400UE, which is in turncontrolled by a temperature-pressure cascade loop, to be described inmore detail. A product temperature sensor 242, located near the end ofholding tube 226, senses the product temperature. A signal representingthis temperature is applied via a line 265 to a firstcontroller-recording 243, such as a conventional Foxboro Model 44/BPunit. This unit includes an adjustable set point reference 245 which isinitially set (manually, for example) to the desired product sterilizingtemperature of the system. The output of the controller recorder 243 isan error signal representing the difference between the set pointtemperature and the actual temperature measured by the sensor 242. Thisoutput signal is applied to a second conventional controller recorder247, preferably identical to the unit 243, serving as the set pointinput thereof. The second controller recorder may be characterized as apressure controller while the first may be characterized as atemperature controller.

A conventional pressure probe 238 monitors the steam pressure deliveredto the sterilizer and applies a corresponding signal to the secondcontroller recorder 247 as an input thereto. An error signalrepresenting the difference between the pressure signal on line 240 andthe output of controller recorder 243 is produced by controller 247 andapplied via a line 249 to the valve 236 to regulate steam flow into thesterilizer.

The illustrated system utilizing two cascaded controller recorderseliminates instabilities that occur if only one controller is used. Asan alternative to the illustrated system, the desired sterilizing steampressure can be used as the set point reference 245 and the pressuresignal on line 240 can be supplied to the first controller 243, whilethe temperature signal on line 265 can be supplied to the secondcontroller 247. This arrangement works equally well.

If the lower conical section 180 of the sterilizer is to be air cooled,a low-pressure feed of ambient temperature is supplied to the inlet 188through conventional equipment including a pressure regulator 244, apressure gauge 246, and a remote control valve 248. The cooling airsupply may be replaced by a cooling water supply with equivalentregulatory components designed for handling water flow. Similarly, asmentioned earlier, the use of a Teflon cone 192 inside the conicalregion 180 can eliminate the need for the air or water cooling network.

An additional air supply is used to force liquid through the holdingtube 226 and the rest of the system during cleaning or during coolingdown of the system when steam is not being used. This supply may be aconventional one-half inch air line 250. This air inlet can be joined tothe steam line 196 at any point between the steam control valve 236 andthe sterilizer 176. The air inlet line should be provided with a checkvalve 252, a pressure gauge 254, a remote control valve 256, and apressure regulator 258.

Product for treatment in the sterilizer 176 is supplied via a mainsupply line 266 which is coupled to a plurality of supply pipes 52, eachfeeding one of the film forming heads 54. In FIG. 23 each of the supplypipes 52 is shown as having a manual valve 262 to provide individualflow adjustment to each of the film forming heads. These manuallyadjustable valves may be eliminated simply by utilizing the distribution"tree" concept illustrated in FIGS. 15 and 16. The main supply line 266includes an input product temperature sensor 264 and a product linecheck-valve 267 which is placed in the main supply line just before thetemperature sensor.

A liquid level sensor 268 may be used to monitor the level of product atthe bottom of the sterilizer 176. As shown, the device is preferably anon-contacting conventional magnetic or gamma ray device including anenergy projector 270 and an energy sensing device 272. Similarly, anoptional non-contacting flowmeter 274 of conventional design may becoupled to the holding tube 226 to monitor the flow of product throughthe holding tube. The holding tube itself is a sanitary line fortransferring the product from the sterilizer 176 to the vacuum chamber224. The residence time of the fastest-moving particle of product in theholding tube is considered to be the holding tube time. A removableorifice 276 is installed at the end of the holding tube where it entersthe vacuum chamber 224. The orifice 276 serves as both an expansionvalve and a control of the flow rate through the holding tube. The sizeof the orifice 276 is established experimentally by operating the systemat different known flow rates and observing the liquid level in thesterilizer after the system has stabilized. For a flow rate of 3,000gallons per hour, for example, an orifice of approximately one ich isused. Similarly, for a flow rate of 600 gallons per hour, an orifice ofapproximately 3/8 inch is used.

The proper size orifice will maintain a constant product level at thebottom of the sterilizer in the outlet pipe 182. Since the flow rate ofa liquid through an orifice is effected by its specific gravity, theliquid level in the outlet pipe 182 of the sterilizer will changeslightly if the specific gravity of the liquid changes. Therefore asupply of variously sized orifices will be required if products withwidely differing specific gravities are to be processed in the system ata constant flow rate.

It is noted that the temperature sensor 242 is used in conjunction withthe controller recorder 243 to record the legal holding tube temperatureand to activate a flow diversion valve (described in the discussion ofFIG. 24), located elsewhere in the processing system if legaltemperatures are not maintained. Another heat sensor 282, which is anindicating thermometer, is positioned adjacent to the end of the holdingtube for sensing the product sterilization temperature and can bevisually checked.

In the vacuum chamber 224, the temperature of the milk is virtuallyinstantaneously lowered to about 160° F. at a vacuum of approximately 20inches of mercury. This rapid reduction in pressure causes removal ofall of the absorbed steam and returns the processed liquid to itsordinary concentration. More importantly, the reduction in temperatureof the product, particulary where milk is concerned, reduces thesensitivity of the product to taste distortion which could be caused byextensive physical perturbations or agitation. Thus, once it is cooledin the vacuum chamber, handling of the milk product becomes lesscritical. However, it is noted that the product must pass at hightemperature through the outlet pipe 182 and the holding tube 226 beforeit reaches the vacuum chamber. Thus handling of the milk as it passesthrough the holding tube is also critical since flavor distortion caneasily occur in the holding tube itself. Furthermore, the flow ofprocessed product through the holding tube must be very closelymonitored to prevent either a buildup of excess product in thesterilization chamber or a drop in the level of fluid in the outlet pipe182. An accumulation of excess material in the sterilization chamber canresult in splashing, and the resultant undesirable physical agitation ofthe product at the bottom of the sterilization chamber as well asburning on of droplets of splash material that reach hotter portions ofthe sterilization chamber wall. Furthermore, if the product is notsteadily withdrawn from the sterilization chamber, its time of treatmentat high temperature increases and accordingly flavor distortion canoccur due to excessive high temperature exposure (i.e., overheating ofthe product). Fluctuations in the level within the sterilization chambercan thus lead to non-uniformity in the resultant product which is veryundesirable from the quality control standpoint.

If, on the other hand, the level of fluid drops too low in the outletpipe 182, steam bubbles may be trapped in the outlet pipe and theholding tube 226. Such steam bubbles affect the holding time and causeit to become unpredictable, again creating the possibility ofnon-uniformity in the treated product. The same steam bubbles alsocollapse unpredictably and cause localized heating of the product andexcess deposits on the walls of the holding tube 226. Such deposits canreduce the diameter of the holding tube and thus further restrict flowleading to a continual backup of fluid within the sterilization chamber,and consequent further loss of quality in the product being processed.Burning on of milk solids to the walls of the holding tube can alsoresult from the lack of a steady flow of product (i.e. a brief delay inpassing through the holding tube). Again, deposits may be created on thewalls of the holding tube further reducing flow and also imparting aburnt flavor to the milk product as it emerges from the holding tube.For all of these reasons, it is essential to accurately control thefluid level at the bottom of the sterilization chamber (or top of theoutlet pipe 182) and to control the flow rate through the holding tube226.

Accordingly, a very precise system is necessary for controlling thefluid level at the bottom of the sterilization chamber and forcontrolling fluid flow through the holding tube 226. In addition tobeing accurate and reliable, however, such a control system must also besuch that it does not engage the hot fluid product, can be kept sterilewith little or no difficulty, and can be produced at a cost which is notprohibitive. To meet all of these criteria, a unique method andapparatus was developed for maintaining the fluid flow and fluid levelin the system of the invention. The unique apparatus relies uponmaintaining a pressure equilibrium and is characterized as a "balancedforce" technique.

In developing this technique it was first determined by extensiveexperimentation that the optimum fluid level was a level at the junctionbetween the bottom of the sterilization chamber 176 and the top of theoutlet pipe 182, as indicated at 284 in FIG. 23. Maintaining the level284 results in a liquid seal at the bottom of the sterilization chamberprohibiting the escape of steam or steam bubbles into the holding tube.It further essentially eliminates the possibility of splashing withinthe sterilization chamber and results in a steady flow of materialthrough the holding tube 226.

The balanced force control is established by adjusting a valve 267 to anappropriate setting so that a desired flow rate of product is introducedinto the sterilization chamber 176. Once this setting is known for agiven product, the valve 267 may be replaced by an orifice plate or thepiping may simply be sized to produce the desired rate at all times.Steam must then be introduced through the supply pipe 196 at anappropriate temperature and pressure to provide adequate heating of theproduct. The orifice 276 is then set to maintain the desired liquidlevel 284 at the bottom of the sterilization chamber. This level ischecked by the use of the liquid level sensor 286. It has beendiscovered empirically that for a given sized orifice 276, a singleliquid level is established in the sterilization chamber 176 when allother conditions remain constant as one would expect. It was alsodiscovered that large system variations did not significantly change theliquid level and moreover did not cause instability in system dynamics.This was completely unexpected. It is a significant finding since leveland flow control in the sterilizer and holding tube are critical to thefilm formation and to preventing overheating and flavor distortions.This finding meant that a fixed orifice is all that would be needed toaccurately control the level in the sterilizer and the flow rate throughthe holding tube.

As an alternative to adjusting the orifice 276, a valve may be installedin place of the orifice 276 and adjusted to the proper flow rate. Oncethe proper rate is established for a particular system, the valve can beremoved and replaced by an orifice permitting the same flow rate.

Data supporting the operation of the force balance method is set forthin TABLE 3. As indicated in the table, it has been observedexperimentally that a very stable flow rate was established when theforce balance level control method was used as opposed to using aconventional feedback control system for modulating the holding tubeback pressure. This was observed using a conventional magnetic flowmeter 274 with readings recorded on a conventional circular chart. Asseen from TABLE 3, significant variations in system parameters, such asflow rate, sterilizer pressure, and temperature do not cause instabilityor loss of the liquid level. Moreover, the change in the liquid level isvery small (less than 2") even when large variations occur in the flowrate and sterilizer temperature and pressure as seen in TABLE 3. It isthus accordingly seen that a stable configuration is established.

The most important and critical aspect of this "balanced force" methodis that large changes in system dynamics that would likely occur duringa commercial operation do not cause instability in the liquid level dueto a balancing of system forces. For example, suppose an orifice issized and placed at the end of the holding tube to provide the properlevel. If the flow rate is increased by 20%, one would expect the levelto continuously rise and fill the sterilizer. This does not occur. Infact, an increase in the flow rate to the sterilizer results in a verysmall increase in the liquid level which again becomes stable. Theincreased flow rate requires additional steam, which requires additionalpressure, which forces more product at the outlet of the sterilizer,thus counteracting the increased flow input. The discovery of thestability of this method is critical to system operation.

To prevent flavor distortion due to contact between the heated productand a metal surface, the entire inner surface of the outlet pipe 182 andthe holding tube 226 may be coated with an appropriate inner materialsuch as Teflon, or the holding tube may be formed of an inert materialsuch as glass.

                  TABLE 3                                                         ______________________________________                                        Balanced Force Level Control Method Data                                      (Orifice 276 = 3/8")                                                                        RUN                                                             PARAMETER       1         2       3                                           ______________________________________                                        Flow Rate       10.7      8.1     9.7                                         (GPM)                                                                         Sterilizer Pressure                                                                           37        21      28.5                                        (psig)                                                                        Sterilizer Temperature                                                                        285       248     270                                         (°F.)                                                                  Liquid Level    53        60      70                                          (0-100 = 6")                                                                  ______________________________________                                         Sterilizer Inlet Temperature was constant at 159° F.                   Flash Chamber Vacuum was constant at 21" mercury.                        

A "pop-off" or maximum pressure valve 286 may be coupled to the steamsupply pipe 196 as a simple and effective way of ensuring that steampressure does not rise above a predetermined value. This valve preventsthe steam pressure from rising in the sterilization chamber 176 and thusmaintains the chamber pressure below a specified maximum. If the steamsupply should increase above a specified maximum, flow would increasethrough the holding tube above a specified limit and the level 284 woulddrop below the optimum position. The pop-off valve 286 provides a devicefor preventing this situation from developing.

It should be pointed out that the balanced force level control methodworks similarly with non-condensable gases, such as air. This is veryuseful in cleaning closed vessels within the present system. Moreparticularly, a selected air pressure is maintained in the sterilizationchamber which will result in a constant level and flow rate. Theconstant level improves the ability of the pressure vessel to be cleanedand eliminates the need for a pump at the discharge or bottom end of thepressure vessel.

The design of the holding tube 226 for an ultra-high temperature (UHT)system in accordance with the present invention is particularly criticalsince many of the flavor distortions which have been eliminated in theunique design of the sterilization chamber 176 can be reintroduced intothe product by various effects occurring within the holding tube. It isparticularly necessary to avoid agitation and turbulence in the holdingtube as oiling off and fat separation can then occur. A very smooth andcontinuous rate of flow through the holding tube is essential to productquality and uniformity. Thus even a negative feedback control networkwhich might be coupled between a valve placed at the position of orifice276 and the flow meter 274 or level detector 268 might causeoscillations in the flow rate or other variations in the flow rate whichcould introduce turbulence and undesirable pressure variations into theholding tube. The balanced force method, on the other hand, permits atotally fixed system to be produced wherein the possibility of flow rateand pressure fluctuations is virtually eliminated.

Concerning holding tube design, it is pointed out that there are threecritical operations in UHT processing where the product can be severelydamaged; heating to ultra-high temperatures, holding at ultra-hightemperatures, and cooling from ultra-high temperatures. Using thesterilizer 176 of the invention, the potential damage to the productduring the heating operation is reduced, and flash cooling in the vacuumchamber 224 which is nearly instantaneous, minimizes product damageduring the cooling operation. Further discussion of the design of theholding tube so as to minimize damage to the product is required.

Because of the viscosity, product particles move through a holding tubewith different velocities and hence different resident times. Thegreatest variation in residence times of product particles occurs withlaminar flow. The average velocity through a pipe is:

    V.sub.avg =4Q/πD.sup.2                                  (1)

and the Maximum Velocity is: ##EQU3## where Q is the flow rate.

Since V_(max) =L/t in a pipe, the length of the pipe for t secondsresidence time of the fastest moving particles then becomes: ##EQU4##where Q is in cubic inches, converting the flow into gallons per secondwe obtain the desired equation for determining the holding tube length:

    L=588.24(Qt/D.sup.2)                                       (4)

Where:

L=holding tube length (inch)

Q=flow rate (gallon per second)

t=holding time (second)

D=inside diameter of holding tube (inch)

Using the above equation, holding tube length for a one second holdingtime was calculated for various tube diameters:

                  TABLE 5                                                         ______________________________________                                        HOLDING TUBE LENGTHS FOR UHT PROCESSING                                       WITH A PUMPING RATE OF ONE GALLON PER                                         SECOND                                                                        time = 1 sec.                                                                            1        11/2   2      21/2 3                                      ______________________________________                                        Holding Tube                                                                             723      300    168    105  71.4                                   length (In.)                                                                  ______________________________________                                    

Lengths of holding tubes of various diameters can now be calculatedusing table 1 and the following formula:

    A=B×C×D                                        (5)

Where:

A=holding tube length (inch)

B=measured flow rate (gallon per second)

C=holding tube length from Table 1 (inch)

D=holding time (second)

EXAMPLE 1

Calculate the holding tube length for a 600 gallon/hour flow rate and a2 second minimum holding time using a 11/2 inch holding tube. Usingequation (5) and the table: ##EQU5## A=100 inches or 8.33 feet

EXAMPLE 2

Calculate the holding tube length for a 3,500 gallons/hour flow rate anda 2 (two) second minimum holding time using a 3 inch holding tube. Usingequation (5) and the table: ##EQU6## A=138.83 inches or 11.57 feet

There are many constraints on holding tube design. Some of the moreimportant constraints are: Bacteriological, Legal, Organeleptic,Orthokinetic Flocculation, Pressure, and Turbulence.

The bacteriological constraint is perhaps the most important and is themajor reason for neating the product. The level of microbial destructiondesired will determine the lower bound on the heat treatment. In theUnited States as well as many other countries, minimum legal standardsare required for pasteurization and sterilization of dairy products.Pasteurization heating requirements are designed to eliminate diseasecausing bacteria while sterilization is designed to inactivate allmicroorganisms and enzymes. The legal U.S. standards for pasteurizationconsist of holding times of 15, 1, 0.5, 0.1, 0.05, and 0.01 second fortemperatures of 161, 191, 194, 201, 204, and 212° F., respectively. Forsterilization, the legal standard consist of a holding time of 2 secondsfor a temperature of 280° F. Note that these are minimal requirementsand may be exceeded without any consequence to the bacteriological orlegal requirements. These constraints, therefore, call for the highestpossible holding time.

While the bacteriological and legal constraints call for a very severeheat treatment, the organeleptic constraint calls for a minimal heattreatment since a severe heat treatment adversely affects the product.Because the rate of microbial destruction becomes much higher than therate of chemical and physical changes as the processing temperatureincreases, the organeleptic constraint calls for a minimal holding timeat the highest possible temperature.

Extensive research at USDA Dairy Products Laboratory indicated thatorthokinetic flocculation plays an important role in the coagulationprocess of UHT treated milk, milk concentrates, and other dairyproducts. The results of the study indicate that high-velocity gradientsin the holding tube at ultra-high temperatures should be avoided. Toreduce these gradients, the holding tube diameter should be as large aspossible. A theoretical determination of the minimal holding tubediameter which will minimize orthokinetic flocculation is: ##EQU7##Where: d=holding tube diameter (cm)

r=particle radius (cm)

η=viscosity (poises)

Q=Flow Rate (cc/sec.)

k=Boltzman's constant (k=1.38×10⁻¹⁶ ergs/degree) and

t=Absolute Temperature (°K.)

For a 3 to 1 milk concentrate the following data were used:

r≦2.5×10⁻⁵ cm

η=0.02 poise

Q=100 cc/sec.

T=410° K. or 278.6° F.

Substituting in (6) one obtains:

d≧2.3 cm (or 0.9 inches)

Allowing a factor of safety of 100% one obtains a value of approximately5 cm or 2 inches.

For fluid milk assuming the following data:

r≦2.5×10⁻⁵ cm

η=0.01 poises

Q=600 gals/hr

T=410° K. ##EQU8## d³ ≧35.49 d≧3.3 cm (or 1.3 inches)

With 100% safety factor

d≧2.5 inches or as large as possible

The Orthokinetic Flocculation constraint, therefore, calls for the useof the largest possible holding tube diameter.

Absolute pressure in the holding tube must be maintained above theproduct saturated vapor pressure to avoid flashing in the tube and thepossible accumulation of non-condensable gases which would cause adecrease in holding time. An absolute pressure 0.6 atm. (8.82 psig)above the saturation pressure in the holding tube is sufficient toeliminate any decrease in holding time. Agitation and turbulence at hightemperatures, particularly in the holding tube should be avoided, sinceit causes oiling-off and fat separation in the processed product. Thepressure constraints, therefore, dictate that an absolute pressure atleast 9 psig greater than the product saturated vapor pressure be usedin the holding tube. Such pressure should not be high enough to causeturbulence. Agitation in the holding tube should also be avoided.

From the preceding discussion it is apparent that the design of theholding tube for UHT processing in accordance with the teachings of theinvention is critical and may have a significant effect on productquality. The tube length should be designed so that the fastest movingparticles will attain the minimum residence time required forpasteurization or ultra-pasteurization. Equation (4) derived from fluidmechanics provides a rationale for calculating the holding tube lengthfor the fastest moving particle, i.e.:

    L=588.24(Qt/D.sup.2)

There are many constraints which must be considered in designing aholding tube, most important of which are the following:

1. Bacteriological and Legal--These constraints determine the minimalholding time and temperature required. Pasteurization calls for aholding time of 15, 1, 0.5, 0.1, 0.05, and 0.01 second for temperaturesof 161, 191, 194, 201, 204, and 212° F., respectively. Sterilizationcalls for a holding time of 2 seconds for a temperature of 280° F.;

2. Organeleptic--This constraint calls for the least possible holdingtime at the highest possible temperatures;

3. Orthokinetic Flocculation--This constraint calls for the largestpossible diameter for the holding tube;

4. Pressure and Turbulence--This constraint calls for an absolutepressure at least 9 psig higher than the product saturation vaporpressure in the holding tube but not high enough to cause turbulentflow. Agitation in the holding tube should also be maintained.

The holding tube 226 of the present invention is designed to meet theseconstraints and provide optimal product quality.

Further discussions of holding tube design may be found in the followingreference materials.

Dickerson, R. W., R. B. Read, and H. E. Thompson. "Performance Tests forPlate Heat Exchangers Used for Ultra-High-Temperature PasteurizationProcesses." Report of Public Health Service, Food and DrugAdministration, Division of Microbiology, U.S. Department of Health,Education, and Welfare. Cincinnati, Ohio, 1968.

Leviton, Abraham, H. E. Vettel, and J. H. Vestal. "Practical Reductionof Orthokinetic Flocculation in Processing of Sterile MilkConcentrates," Journal of Dairy Science. Vol. XLVIII, No. 8 (August,1965), pp. 1001-9.

Nahra, J. E. "The DASI Free-Falling Film Method for the Pasteurizationand Sterilization of Dairy Products." Presented at Ohio StateUniversity. Feb. 14, 1974.

Shames, Irving H. Mechanics of Fluids. New York: McGraw-Hill Book Co.,Inc., 1962.

Stroup, W. H., R. W. Parker, and R. W. Dickerson. "Steam Infusion Heaterfor Ultra-High-Temperature Pasteurization," Journal of Dairy Science,Vol. LV, No. 4 (April, 1972) pp. 537-9.

Attention is now directed to FIG. 24 which illustrates the sterilizer176 of the present invention in conjunction with a complete processingsystem. In the system, the raw input product, such as raw milk, enters abalance tank 288 through a supply pipe 290, to which a water feed pipe292 may also be coupled. The product enters the balance tank 288 atapproximately 4° C. (40° F.). It is pumped out of the balance tank by acentrifugal pump 294, through a conventional flowmeter 296 to aconventional pre-heater 298 where it is heated to approximately 80° C.(176° F.) by water which has previously been used to cool vapors in aflash chamber condenser 300. A variable valve 302 is coupled to theflowmeter 296 through a conventional feedback servo network 304 toregulate the system flow rate at the output of centrifugal pump 294. Aconventional temperature sensor and servo network 306 monitors thetemperature of the product in line 266 and controls the application ofheated culinary steam to the preheater 298 via a valve 308 in accordancewith the product temperature.

The preheated product enters the UHT sterilizer 176 where it is formedinto films, as previously described, and heated to a temperature ofapproximately 143° C. (290° F.). Steam pressure maintains apredetermined level in the UHT heater, in accordance with the balancedforces control network previously described, and pushes the productthrough holding tube 226 into flash chamber 224 where the product isinstantaneously cooled to 82° C. (180° F.). The same amount of steamused in the UHT sterilizer 176 is flashed off in the flash chamber bycontrolling the vacuum therein. In this regard it is noted that hotvapors are drawn off from the flash chamber through a line 310 andsupplied to the condenser where they are condensed by cold watersupplied through a line 312. The cold water is heated in this processand delivered to the preheater 298 via a line 314. A conventional vacuumpump 316 evacuates the flash chamber and condenser.

The cooled product is removed from the flash chamber by a conventionalaseptic product removal pump 318 and is delivered via a line 320 to aconventional homogenizer 322 where a homogenizing pressure ofapproximately 200 kg/cm² (3000 psig) is maintained. A conventionaltemperature control and servo network 324 couples the line 320 with anair valve 326 to control the vacuum within the flash chamber and thuscontrol the temperature of the product delivered to the homogenizer. Theproduct level in the flash chamber 224 is controlled by a by-pass line328 around the homogenizer 322. A check valve 330, controlled by aconventional level sensing and servo network 332 controls the deliveryof product to the by-pass line.

The homogenizer 322 pushes the processed product through a line 334 to aconventional aseptic cooler 336 where cold water from the preheatercools the product from 85° C. (185° F.) to 20° C. (68° F.) for asepticstorage or for direct filling of aseptic packages by means of a seriesof conventional output surge and filler valves 338, back pressure valves340 and 342 maintain a positive pressure in the aseptic product lines tominimize the risk of contamination. A conventional pressure monitor andservo network 344 controls the operation of back pressure valve 340. Aflow diversion valve 346 is controlled by the controller recorder 243 inresponse to temperature measurements of the product within the holdingtube 226. If the temperature in the holding tube falls below thelegally-required minimum. the flow diversion valve is activated todivert the improperly-processed product back to the balance tank via aline 348 and a drain/rerun valve 350 to the balance tank 288 forreprocessing. If the legally required temperatures are maintained withinthe holding tube, the diversion valve remains closed and the processproduct is delivered directly to the surge and filler valves.

It is noted that the system may be completely automated with an initialsterilization cycle using hot water, a product cycle and a subsequentcleaning in place cycle.

The major aspects of the present invention discussed herein togethercooperate to produce results which have long been sought after but havebeen unattainable using prior art technology. These results are theefficient and continuous production of fully sterilized milk which isvirtually indistinguishable from fresh whole milk in taste. Tests onsamples produced by the present system conducted at the University ofMaryland have proven that test samples of milk produced utilizing thesystem of the invention can be stored unrefrigerated for periods up toeight weeks with no significant taste difference when compared withfresh, pasteurized milk. Furthermore, in taste tests held at theUniversity of Minnesota in July, 1977 milk produced in accordance withthe general method of the present invention was compared to regularpasteurized milk and to sterilized milk using conventional technology.The product produced using the present invention received the highestscore of all products indicating taste preference by the panel oftesters. These results confirmed earlier tests conducted in 1976 by theDairy Marketing Forum sponsored by the U.S. Department of AgricultureCooperative Extension Service and the University of Illinois at UrbanaChampaign.

In operation, the apparatus of the invention, which may be characterizedas an ultra high temperature (UHT) sterilizing system, receivespreheated products from an appropriate source. This product is formedinto one or more continuous and fully isolated falling films of product.Virtually any number of independent falling films may be produced in thesterilization chamber 176, depending only upon the size of the chamber.Naturally, sufficient spacing must exist within the chamber to preventinterference among the various films. The falling film is characterizedby the fact that it never engages any surface which is hotter thanitself. It is formed using a distribution head having a plurality ofproperly spaced apertures to maintain careful control over the filmthickness and shape. The falling film is subjected to extremely rapidheating to a temperature in the range of between 280° and 300° F. byfully saturated culinary steam. Special baffling and steam distributiontechniques are used in accordance with the invention to prevent thesteam from disturbing the continuous nature of the falling film. This ishighly significant in the context of the present invention since thefilm must fall to the bottom of the sterilization chamber without beingdisturbed or split into components to prevent taste distortion. Thecareful reduction in steam velocity and ultimate distribution of steamaround the falling film's product prevent the steam from interferingwith the continuous nature of the falling film. Similarly, the height ofthe film is carefully adjusted as is the flow rate of the productforming the film so that the film falls to the bottom of thesterilization chamber without breaking into droplets or otherwisebecoming discontinuous. As such, surface tension holds all particles ofthe film together even as they strike the bottom of the chamber. As aresult no splashing or substantial agitation occurs as the film reachesthe bottom of the chamber and is fed into the outlet pipe. To preventagitation, splashing or other physical disturbance of the fluid in theoutlet and holding pipes, extremely accurate control of the fluid levelat the bottom of the sterilization chamber is required. To meet thisrequirement, and to meet the requirement of maintaining extremely steadyflow through the holding tube and to still preserve the easily cleanablenature of the equipment, a balanced force technique has been developed.The advantage of this technique is that it eliminates expensive controlswhich could contaminate the milk product, could be difficult to maintainin a sterile condition and might be subject to failures ofmalfunctioning which would result in perturbations in the fluid and flowlevel resulting in turn in inconsistencies in the output product. Thebalanced force technique, however, eliminates all of theseinefficiencies simply by controlling the input flow and regulatingoutput flow in such a way that a fixed fluid level is found andmaintained to keep the system fully stable and operational withvirtually no risk of failure or product distortion.

The principal advantages of the continuous, isolated falling filmsterilization method and apparatus of the present invention can besummarized as follows:

Flavor--Product (e.g., milk) flavor as good as or better thanpasteurized. Chalky, sandy or burnt flavors associated with UHT milkeliminated.

Consistency--Because of the inherent design of the system, productquality is consistent throughout the production run.

Minimum Product Damage--Due to the inherent characteristics of the freefalling film UHT heater minimum product damage results for desiredsterilizing effect. Product characteristics such as fat separation andsedimentation in milk and lack of whipping ability in cream processedwith conventional UHT systems does not occur using the process andapparatus of the invention.

Large Flow Rates--Small as well as large flow rates are possible. Aslittle as 100 gph to more than 5000 gph. This makes large operationseconomically feasible.

Variable Flow Rates--The flow rate of the system can be variedsubstantially ±20% without losing stability. This feature will limit theneed for large aseptic surge tanks which are a high cost item.

Product Variety--System can be used for many products with a wide rangeof physical parameters including viscosity, specific weight, specificheat, heat sensitivity, and others.

Long Running Times--System can be run for a long period of time withoutshutdown. Twenty hour per day operation should be feasible.

Minimum Cleaning--System can be cleaned in place (CIP) automatically.Minimum time is needed because of minimum deposit (burn-on) on hotsurfaces.

Efficient Energy Utilization--The present UHT heater has high heattransfer efficiency (more than 95%); moreover, there is no reduction inheat transfer efficiency as a function of running time.

Large Range of Temperature Increases--A large range of temperatureincreases are possible in the present UHT heater. As little as 20° F.increase to as much as 250° F. increase in less than one third (1/3) ofa second.

Maximum Heat Penetration--Maximum heat penetration is accomplished byuse of the thin isolated, continuous free falling films with saturatedsteam, and very large heat transfer area.

Pasteurizer, Ultra-Pasteurizer, Sterilizer--System has been cleared bythe United States Public Health service as a legal pasteurizer,Ultra-Pasteurizer or sterilizer. Ultra-Pasteurized dairy products inmost states would not need to conform to state dating laws.

Minimum Maintenance--The present UHT heater has no moving parts, it isconstructed of stainless steel and requires little maintenance. Sincethe Ultra-High Temperature portion of the system involves only thesterilizer, minimum maintenance is required in other portions of thesystem. Gasketing of plates or tubular heat exchangers is eliminatedor/and reduced.

Manual or Automated--The system can be fully automated or it can bemanually operated by a trained operator. The level of automation can bedetermined by the user.

The present system can be used for processing and heat treating alltypes of fluent materials. Naturally the characteristics of the materialto be treated must first be studied and fully understood before heattreatment can begin. For example, it is necessary for each product todetermine the appropriate temperature-time relationship for optimumheating. Once this relationship is determined, the present system can beset to process any fluent material according to very precise time andtemperature limitations and with an absolute minimum of physicalperturbation or agitation. To prepare the system for treating any suchgeneral product, once the time-temperature characteristics of theproduct are determined, it is first necessary to set the height of thefalling film in accordance with the required heating time. Raising theheight of the falling film increases the time exposure of the product toheat, while lowering the height of the film reduces the exposure time.Similarly, the temperature and pressure within the sterilization chambermust be set in accordance with experimentally determined optimum valuesfor the product in question. It is then necessary to set the flow rateof the system at an appropriate level. The flow rate is determined bythe width and thickness of the falling film, the number of falling filmsutilized, and by the viscosity of the product. The system can then beadjusted using the balance force technique to operate uniformly at thedesired flow rate.

Obviously, numerous additional modifications and variations of thepresent invention are possible in light of the above teachings. It istherefore to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:
 1. A method for maintaining a stable fluid level in a pressurized fluid processing system including a vessel, independent of large changes in system dynamics, comprising the steps of:establishing a desired flow rate into said system, permitting said desired flow rate to vary independently of a predetermined fluid level at the bottom of the said vessel, pressurizing and heating said system to a desired level; collecting said fluid with said predetermined fluid level at the bottom of said vessel, setting constant parameters for a continuous flow outlet means for said system whereby said predetermined fluid level is maintained in said system; and maintaining said setting regardless of said variations in said flow rate into said system.
 2. An apparatus for treating fluent material with a heated gas having an initial flow velocity comprising:a pressure vessel, dispensing means mounted within said vessel for forming at least one isolated, continuous film of said fluent material, baffle means mounted within said pressure vessel for reducing said initial flow velocity of said heated gas and for distributing said gas within said vessel to achieve rapid heating of said isolated, continuous film of said fluent material while subjecting said film to minimum of physical perturbation, fluid outlet means coupled to said vessel for withdrawing treated fluent material therefrom; holding tube means coupled to said fluid outlet means for maintaining said fluent material at a selected temperature for a selected time interval; and fluid collecting means within said pressure vessel for intercepting said isolated, continuous film and for supplying said treated fluent material to said outlet means with a minimum of physical agitation; wherein said dispensing means comprises: a structure forming a chamber for receiving a quantity of said fluent material, said structure having a discharge aperture formed therein; and, distribution means positioned within said structure for distributing substantially equal quantities of said fluent material to each segment of said discharge aperture; and wherein the cross-sectional areas of said outlet means and holding tube means are fixed so as to provide balanced force control means for maintaining a stable liquid level in said pressure vessel independent of changes in system dynamics.
 3. The apparatus of claim 2 wherein said distribution means comprises:a structural member having a non-uniform distribution of apertures therethrough, said apertures distributed to form a varying net opening through said structural member.
 4. A dispensing head as in claim 3 wherein:said distribution of apertures includes a series of openings of progressively smaller size.
 5. A dispensing head as in claim 3, wherein:said distribution of apertures includes a plurality of openings of similar size spaced to provide a progressively smaller density of openings through said structural member.
 6. A dispensing head as in claim 3, wherein said distributing means comprises:a generally flat plate dividing said chamber within said elongated structure into two sub-chambers.
 7. A dispensing head as in claim 3, wherein said distributing means comprises:a tubular member disposed within said elongated structure and extending along the axis thereof.
 8. An apparatus as in claim 2, wherein said dispensing means comprises:supply pipe means for carrying a flow of said fluent material, branch pipe means coupled to said supply pipe means for dividing said flow therein into a plurality of components; and a plurality of film forming head means coupled to said branch pipe means for forming a plurality of thin, isolated, continuous films of said fluent material.
 9. An apparatus as in claim 2, wherein said fluid collecting means comprises:a conical lower section of said pressure vessel, a layer of non-metallic, chemically inert material of low thermal conductivity covering said conical lower section; and, means associated with said conical lower section for preventing excessive heating thereof.
 10. An apparatus as in claim 2, wherein said fluid collecting means comprises:a conical element formed of a non-metallic, chemically inert material of low thermal conductivity supported within a lower portion of said pressure vessel. 